Hemopoietic stem cell concentrates have been prepared from normal human peripheral blood. Current studies evaluating this material involve: 1. Attempts to clone the stem cells in vitro and in vivo; 2. Assessment of transplantability of these cells in vitro and from in vitro to in vivo cultures, as a measure of their potential self-replication; 3. Investigation of cell-cell interaction involving stem cells and syngeneic thymus-derived lymphocytes; 4. Estimation of the role of steroid receptors in hemopoiesis; and 5. Labeling experiments with 51 Chromium, H3-Thymidine, H3-Uridine, and H3-Glucosamine to define the stem cell metabolism and progeny in culture.